I will respond from my experience, and of course, Todd has a much broader base of experience and knowledge.
Stocking rate depends somewhat on objectives, as usual, for the pond. If the pond has a bad algae problem, then higher rates would be advised. If very clear, then also higher rates and possibly larger sizes would be advisable due to predation. I started with 10 to 20 pounds per acre the first year...smaller ponds at 10 pounds, larger ponds with larger bass at the higher end. I dropped down to 5 pounds per acre for small ponds this year(small being 2 or less acres) and received complete algae control and forage production. I think the previous years experience helped in regard to algae control. In my larger pond, I held to about 10 pounds per acre.
Its kind of like how much of a good thing do you want or can afford. A 45 acre pond/lake, even at 10 pounds per acre, is a bunch of Tilapia.
Size...this is really personal opinion based on experience. I believe, unless you are raising them for personal consumption, the smaller sizes are actually preferable (4 to 6 inch) over the 6 to 8 inch or larger size. A 4 inch Mozambique Tilapia is sexually mature, will begin reproducing immediately. To me, two 4 inch Tilapia are preferable over one 8 inch Tilapia for forage production and algae control. Of course, the other side of the equation is predation. In my experience, the reproductive capabilities of the Tilapia overcome predation limitations and larger sizes are not required. That may change for me now that LMB and HSB are getting to be very large in my largest pond.
My bottom line: first year 10 to 20 pounds per acre of 4 to 6 inch fish and 5 to 10 pounds in succeeding years depending on results. That's based on two years experience with 4 ponds stocked with Mozambique Tilapia.
